Recently, progress has been made in the development of a display device (EL display device) employing a light-emitting element (hereinafter referred to as EL element) which uses EL phenomenon of a light-emitting material. An EL display device does not need a backlight unlike a liquid crystal display because the light-emitting element itself has an ability to emit light. In addition, an EL display device has the advantages of wide viewing angle, high contrast, and so on.
It is said that an EL element emits light by releasing energy when electrons injected from a cathode and holes injected from an anode are recombined at a luminescence center in an organic compound layer to form a molecular exciter and when the molecular exciter moves back to the ground state by applying a voltage with interposing the organic compound layer between a pair of electrodes. Singlet excitation and triplet excitation are known as an excitation state, and it is considered that light-emission can be obtained through both the excitation states.
Moreover, there are an inorganic light-emitting material and an organic light-emitting material as a light-emitting material which is used for an EL element, and an organic light-emitting material that requires a low driving voltage attracts attention.
However, after driving a certain period of time, an organic EL element using an organic material for the EL element has a problem in that light-emitting characteristics such as luminance and luminance uniformity degrade drastically compared to those at the beginning. The low reliability is the main factor for the limited practical application.
Moisture and oxygen which enter the organic EL element from outside are cited as one of the factors for deteriorating the reliability.
In the EL display device (panel) using an EL element, the moisture which enters an inner part leads a serious degradation in reliability to cause a dark spot, shrink and a degradation of luminance from a peripheral part of a light-emitting device. The dark spot is phenomenon in which luminance decreases partially (including nonluminescent element), and it occurs when a top electrode has a hole. In addition, the shrink is phenomenon in which luminance degrades from a pixel end part (edge).
The development has done on a display device having a structure which prevents the above degradations of an EL element. There is a method for storing an EL element in an airtight vessel to confine the EL element within an enclosed space and further providing drying agent in the enclosed space with isolating from the EL element (for example, see Patent Reference 1).
(Patent Reference 1)
Japanese published unexamined application Hei 9-148066
In addition, there is a method for forming a sealing agent on an insulator where an EL element is formed and filling an enclosed space surrounded by a cover agent and the sealing agent with filler including resin and the like to be shut out from an outer part (for example, see Patent Reference 2).
(Patent Reference 2)
Japanese published unexamined application Hei 13-203076
According to the above patent references, a sealing agent is formed on an insulator where an EL element is formed thereon. Then, an enclosed space is formed, which is surrounded by the sealing agent and a cover agent by using the sealing agent. This sealing step is conducted in an inert gas atmosphere; therefore, there is a minute amount of moisture and oxygen in an inner part of a display device in the immediate aftermath of the sealing, not a large amount of moisture and oxygen from the beginning.
That is, moisture that is to be a cause for a degradation such as a dark spot enters inside of a display device after sealing in many cases. In addition, an insulator and a cover agent are often metal or glass; therefore, moisture and oxygen enter from the sealing agent.
FIG. 1 is a top view of the EL display described in Patent Reference 2. Reference numeral 401 shown by a dotted line denotes a source side driving circuit; 402, a gate side driving circuit; 403, a pixel portion; and 409, an FPC (flexible print circuit). In addition, reference numeral 404 denotes a cover agent; 405, a first sealing agent; and 406, a second sealing agent. FIG. 27 is a cross-sectional view of a conventional EL display device like that of FIG. 1 (the second sealing agent 406 is not shown). As shown in FIG. 27, in a region A of a sealing region, an EL element is encapsulated in an inner portion by using a sealing agent.
According to Patent Reference 1 and Patent Reference 2 above, the sealing agent shuts the EL element off from external moisture like this manner in the region A of the sealing region of FIG. 27.
In the case of the structure in which an EL element is stored in an airtight vessel as described in Patent Reference 1, an EL display device grows in size by the size of the vessel. Moreover, the drying agent is arranged separately from the EL element to prevent a negative effect by stacking a drying agent (protective layer including the drying agent) directly on the EL element. Accordingly, the airtight vessel grows further in size. Although the size of the EL display device increases, the size of a light-emitting portion does not change. Here, an advantage of a thin shape is not taken, which is needlessness of a backlight of the EL display device. Further, in the structure of Patent Reference 1, there is a risk that moisture which enters the airtight vessel comes into contact with the EL element to lead a degradation of EL element because the drying agent absorbs moisture in the airtight vessel.
In Patent Reference 2, a filler such as resin shuts an EL element off from external moisture. However, increase in the size of the EL display device is inevitable because the sealing agent is applied to the region A in FIG. 27 to fabricate an enclosed space.
As described above, the area of a part which dose not emit light increases when a display device area except a pixel portion emitting light (region A in FIG. 27) is large. Accordingly, the increase in the size of the display device is required in order to obtain the same size of light-emitting portion.
In order to solve this problem, a method for applying a sealing agent onto an insulating layer such as an interlayer film and a protective film. FIG. 17 shows the EL display device above, and FIG. 2 shows an enclosed end part of a sealing region at an end. In FIG. 2, reference numeral 21 denotes a substrate; 22, a counter substrate; 23, a gate insulating film; 24 and 25, interlayer films; 26, a wiring; and 27, a sealing agent.
As shown in FIG. 2, in the sealing region, the gate insulating film 23 on the substrate 21, the interlayer films 24 and 25, and the wiring 26 are sequentially stacked, and the sealing agent 27 is applied on the insulating layer (laminated films). In this structure, it is possible to reduce the size of the region A in FIG. 27 which dose not emit light. FIG. 2 is an example; the materials of layers which are stacked on a TFT substrate side and the sequence of the laminated layer are not limited to this example. Here, as an example, a display device has a structure in which a base film, a gate insulating film, a protective film, an interlayer film and a wiring are sequentially stacked on a glass substrate and which has the wiring on the top of those films.
However, in the case where a sealing agent for sealing is on a laminated films as shown in FIG. 2, all of the laminated films are exposed directly to the atmosphere outside of the panel. Accordingly, moisture and oxygen which are outside of the panel enter through the laminated films into the display device. Further, in the case that a material having high moisture permeability such as acrylic is used as the interlayer film, the moisture and the oxygen which enter further increase.
Moisture and oxygen enters from this acrylic serving as the interlayer film and top and bottom interfaces of the acrylic, and reach the interlayer film adjoining to an EL element through a disconnecting part due to inferior film formation of a source and drain electrodes in a contact hole. Accordingly, various degradations such as contamination of the inner part of the display device, a degradation of electric properties, a dark spot and shrink are caused.
Therefore, an object of the present invention is to provide an EL display device of high reliability and its manufacturing method by preventing moisture and oxygen, the factors for degrading properties of an EL element, from entering without increasing the size of the EL display device.